Endoscopes have recently come into extensive use in the medical and industrial fields. Recent medical endoscopes do not have an insertion member and there is no longer an insertion process. These are medical endoscopes are encapsulated within a capsule, which a patient can swallow. This eliminates the pain associated with insertion of prior art endoscopes that have an insertion member. Examples of capsule endoscopes include, for instance, those disclosed in the Japanese Laid-Open Patent Application 2001-91860 and the patent publication PCT WO 00/76 391 A1.
The prior art capsule endoscope disclosed in Japanese Laid-Open Patent Application 2001-91860 is provided with an objective lens and an illumination means consisting of light emitting diodes symmetrically located in relation to the objective lens within a nearly semi-spherical transparent cover. Part of the object is illuminated by the light emitting diodes and imaged by the objective lens onto an image sensor for observation. The prior art capsule endoscope disclosed in patent publication PCT WO 00/76 391 A1 includes a single, oval dome, optical window. An illumination element and a receiving element are positioned above or in contact with the focal curve plane of the oval dome. Plural illumination elements are positioned on the focal curve so that light from the illumination elements returns to some other point on the focal curve when a portion of the illumination light is reflected by the inner and outer surfaces of the window. Therefore, the receiving element is positioned somewhere other than on the focal curve in order to prevent light that is reflected at the interfaces of the oval dome surface from entering the receiving element, thereby preventing flare and ghosting that adversely affect the proper detecting of images.
The prior art capsule endoscope disclosed in Japanese Laid-Open Patent Application 2001-91860 does not describe a means to prevent or reduce flare and ghosting caused by a portion of the illumination light from the illumination means entering the objective lens after it has been reflected at air interfaces of the transparent cover. The prior art capsule endoscope disclosed in patent publication PCT WO 00/76 391 A1 uses an oval dome, transparent cover for the illumination and observation window, which is more costly to produce than a semi-spherical transparent cover. Furthermore, plural illumination elements are positioned on the focal curve. Since each element should be adjusted in position, this design requires additional labor.
When the illumination elements are light emitting elements (LEDs), the illuminating elements have a non-insignificant size. Therefore, in order to position the LEDs on the focal curve, the focal curve must be sufficient in length to accommodate the area in which the LEDs are to be positioned. This causes the size of the oval dome to become larger, which disadvantageously requires that the capsule be larger. However, increasing the size of the capsule is undesirable because it becomes difficult, even painful, to swallow such an encapsulated endoscope. Thus, the advantage of using a capsule endoscope is lost. Accordingly, the arrangement of the illumination means and the image detecting element within a capsule must be designed in a manner whereby the capsule can be made as compact as possible.